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Abstract

We report direct observation of the 2D transverse near-field intensity and polarisation distribution of surface plasmon polaritons guided on metal nanowires. Quadrupolar modes are excited on an array of coupled nanowires arranged around the central glass core in a photonic crystal fibre, with lobes whose orientation depends on the polarisation state of the launched core light. The radial electric field is resolved using a polarization sensitive near-field probe in light-collection mode.

Figures (6)

Design of the device. (a) Schematic of the structure. (b) SEM of the polished cross-section of the sample with six rings of gold nanowires, making 120 in total. Dark grey is silica and light grey is gold.

Operation of the gold filled PCF (a) Calculated intensity distributions (vertical polarisation of the core light) at 840 nm for: the quadrupolar SPP mode of an isolated nanowire (left, the white dashed circle indicates the gold-silica interface), the even core-pSM mode (centre) and the odd core-pSM mode (right). The scale-bar corresponds to 0.5 µm for the left-hand panel and 3 µm for the centre and right-hand panels. (b) Refractive index difference (real part) between the simulated even and odd core-pSM modes (blue and black curves) and the glass core mode in the empty PCF (grey curve): Δn = (nSM – nempty). The dashed red curve shows the same quantity for an isolated m = 2 SPP mode; it lies on top of the curve for the even core-pSM mode and cuts off at ~845 nm (red circle). The green dashed vertical line marks the wavelength (785 nm) of the laser diode used in the SNOM experiments. (c) Attenuation of the even and odd modes and of the isolated m = 2 SPP. The odd core-pSM cuts off at 890 nm. (d) Experiment (purple) and modelled attenuation (orange). The dashed purple horizontal line indicates the limit of the dynamic range of the optical spectrum analyser used.

Solutions of the nearest-neighbour coupling model for 18 nanowires plus core. The numbers represent the percentage of modal power at each site, the color code represents the phase (red is zero, black is π): (a) the quasi-even supermode (all nodes approximately in phase); (b) the quasi-odd supermode (each ring approximately π out of phase with its neighbour).

Experimental setup, SNOM results and FE-modeling of the near-field without analyser. (a) Sketch of the experimental set-up for the SNOM measurements (for details see text). LD: laser diode, P: polariser, λ/2: half-wave plate, O1-O2: microscope objectives, C: cantilever, A: analyser, L: lens, I: iris, APD: avalanche photo-diode. Experimentally measured near-fields and simulated FE results at 785 nm for (b) vertical and (c) horizontal polarisation of the PCF core mode (indicated by the white double-headed arrows). The white dashed circles show the gold-silica interface and the white dashed squares indicate the position of the nanowires. The purple arrows on the FEM plots show the direction of the local transverse electric field at a fixed moment in time (upper and lower plots taken together). The color bars show the correct scale for the lower images only. The upper images are saturated for a better contrast.

Polarisation-resolved near-field measurements and FE results at 785 nm for vertical input polarisation. The logarithmic intensity profiles of the modes are shown for two orientations of the analyser relative to the vertical input polarisation: (a) parallel configuration (b) orthogonal configuration (indicated by the yellow double-headed arrows). The modes shown in this figure correspond to the one shown in Fig. 4 (white dashed circles/squares, white double-headed arrows and color scale are the same).

Spatially resolved polarisation state of the near-field of a quadrupolar SPP mode. (a) The white double-headed arrows show the orientation of the electric field vectors (90 nm spacing) for the SNOM measurement, their lengths being proportional to the absolute value of the transverse electric field. For clarity, no arrows are shown at regions where the intensity is low. The red double-headed arrow shows the input polarisation state. (b) FE-simulations showing the local transverse electric field at a fixed moment in time. The underlying intensity patterns are the same as the two upper left-hand images in Fig. 3(b).